1. Field of the Invention
The present invention relates to a mold opening/closing control apparatus for an injection molding machine.
2. Description of the Related Art
In a conventional injection molding machine, resin which is heated in a heating cylinder and fluidized is injected into a mold at a high pressure, cooled therein so as to solidify or harden, and then the mold is opened and a molded product is removed. The injection molding machine has a mold clamping apparatus by means of which a mold is opened or closed and mold clamping is performed so that molten resin will not leak during the time of injection.
There are two types of the above-described mold clamping apparatus: a toggle type mold clamping apparatus in which a force generated by a hydraulic cylinder or a motor is amplified by a combination of links of toggle joints, and thus a large mold clamping force can be obtained; and a straight hydraulic type mold clamping apparatus in which a clamping force is directly generated by oil supplied to a mold clamping cylinder.
A mold clamping cylinder and a mold opening/closing cylinder are disposed in the straight hydraulic type mold clamping apparatus. From the necessity of improving molding efficiency, mold opening/closing is performed at a high speed in a mold opening/closing cylinder. However, when mold closing is performed at a high speed, a movable metal mold could strike a fixed metal mold due to the inertial force of the movable platen, the movable metal mold or the like when the mold closing is terminated. When mold opening is performed at a high speed, a movable platen could strike a mold clamping cylinder by an inertial force similar to that described above. Accordingly, position control for a mold opening/closing cylinder is performed by a hydraulic control valve.
That is, an electromagnetic change-over valve for selectively supplying oil to either of the oil chambers of the mold opening/closing cylinder is disposed. A variable capacity pump is connected to one of the oil chambers of the electromagnetic selecting valve, and an electromagnetic proportional flow-rate control valve is connected to the other oil chamber. Thus, by controlling the variable capacity pump and the electromagnetic proportional flow-rate control valve, position control for a mold opening/closing cylinder is performed.
In this case, a certain degree of mold releasing force is required during an initial period of releasing the mold. A maximum discharge amount of the variable capacity pump can be determined as described below. That is, the setting of a system pressure determines the pressure area of the mold opening/closing cylinder. Therefore, a maximum discharge amount of the variable capacity pump can be determined by a product of the above-mentioned pressure area and a required maximum speed.
However, a large releasing force is required only during an initial, short period. The load while the mold opening/closing cylinder is moved at a high speed after the mold opening is started is small. Hence, a differential circuit equivalent to the changing of a pressure area can be used. As a result, a variable capacity pump having a small maximum discharge amount can be used, and therefore costs can be reduced.
FIG. 1 is a view showing a mold opening/closing control apparatus in which a differential circuit is used.
In FIG. 1, reference numeral 31 denotes a cylinder in which a mold clamping cylinder and a mold opening/closing cylinder are made in one piece (hereinafter referred to as a mold clamping cylinder). A piston 32 is slidably disposed inside the cylinder. A large-diameter rod 33 and a small-diameter rod 34 are projected and formed on both sides of the piston 32. The large-diameter rod 33 is connected to a movable metal mold via a movable platen (not shown). The piston 32 reciprocates by the operation of the mold clamping cylinder 31, and mold opening/closing and mold clamping are performed.
An oil chamber 35 is formed in the end surface of the small-diameter rod 34 of the piston 32. Oil is supplied to the oil chamber 35 in a case where the movable platen is moved to a fixed platen at the time of closing the mold. An oil chamber 36 is formed in the large-diameter rod 33 of the piston 32. Oil is supplied to the oil chamber 36 in order to separate the movable platen from the fixed platen at the time of opening the mold. In addition, an oil chamber 37 for clamping the mold is formed in the small-diameter rod 34 of the piston 32 and also in the end surface of the piston 32.
Reference numeral 38 denotes an electromagnetic change-over valve by means of which switching is performed by the actuation of solenoids a and b. The ports of the electromagnetic change-over valve 38 are individually connected to the oil chambers 35 and 36 inside the mold clamping cylinder 31, a variable capacity pump 39, an electromagnetic change-over valve 45, and an electromagnetic proportional control valve 40. The electromagnetic change-over valve 38 goes into position I if the solenoid a is driven and into position II if the solenoid b is driven. Position N is a neutral position. At position I, oil discharged from the variable capacity pump 39 is supplied to the oil chamber 35 inside the mold clamping cylinder 31, causing the movable platen to move in the rightward direction via the large-diameter rod 33 and causing the mold to be closed. At position II, oil discharged from the variable capacity pump 39 is supplied to the oil chamber 36 inside the mold clamping cylinder 31 via oil passage 48, causing the movable platen to move in the leftward direction via the large-diameter rod 33 and causing the mold to be opened. At position N, a spool is placed in a neutral position, and the mold clamping cylinder 31 stops.
Reference numeral 39 denotes a variable capacity pump, in which the inclined rotational angle of a swash plate is changed by an instructed signal, whose amount of discharge is changed in proportion to the signal. Reference numeral 40 denotes an electromagnetic proportional flow-rate control valve which is electrically controlled by the solenoid a and by means of which a flow rate is proportionally controlled by an electrical signal. The electromagnetic proportional flow-rate control valve 40 constitutes a meter-out circuit by which control is made possible by the oil chamber 36. By controlling the electromagnetic proportional flow-rate control valve 40, the amount of oil discharged from the oil chambers 35 and 36 inside the mold clamping cylinder 31 is regulated, exerting a braking action on the piston 32 of the mold clamping cylinder 31.
Reference numeral 41 denotes an electromagnetic change-over valve which goes to positions I and II by the actuation of the solenoid a. At position I, the electromagnetic proportional flow-rate control valve 40 is connected to an oil tank 44. At position II, an oil path 42 is connected with the electromagnetic proportional flow-rate control valve 40 through an oil path 43.
The electromagnetic change-over valve 45 goes into positions I and II by the actuation of the solenoid a. The electromagnetic change-over valve 45 is made to go into position II at the time of opening and closing the mold. During that time, oil is taken into or discharged from between the oil chamber 37 and an oil tank 49 through a prefill valve 47 having pilot check. The electromagnetic change-over valve 45 is made to go into position II at the time of closing the mold, and oil is supplied to the oil chamber 37 via oil passage 46 for clamping the mold through the electromagnetic change-over valve 38. The above-described electromagnetic change-over valves 38, 41, and 45 are constructed only so as to be switched, and their resistance to passage is small.
The mold clamping cylinder 31 has the large-diameter rod 33 and the small-diameter rod 34 on opposing sides of the piston 32, as described above. The two of them constitute oil chambers 35, 36, and 37. When pressure areas at the oil chambers 35, 36, and 37 are represented respectively as A.sub.1, A.sub.2, and A.sub.3, pressure area A.sub.3 is a value sufficient to generate a mold clamping force and pressure area A.sub.2 is a value sufficient to generate a mold releasing force. To move the mold, an oil pressure is applied to pressure areas A.sub.1 and A.sub.2. The pressure area A.sub.1 is made half of the pressure area A.sub.2, and a differential circuit is also used at the time of opening the mold.
At the time of releasing the mold by the mold clamping apparatus constructed as described above, the electromagnetic change-over valve 38 is made to go into position II, the prefill electromagnetic change-over valve 45 is made to go into position II, and the electromagnetic change-over valve 41 is made to go into position I. In this state, oil discharged from the variable capacity pump 39 is supplied to the oil chamber 36, whereas the oil in the oil chamber 35 is released to an oil tank 44 via the electromagnetic change-over valve 38, the electromagnetic proportional flow-rate control valve 40, and the electromagnetic change-over valve 41.
Since the pressure in an oil path 48 is high, the prefill valve 47 having pilot check is opened due to a pilot pressure received. The oil inside the oil chamber 37 is released to the oil tank 49 through the prefill valve 47 having pilot check.
At the time of opening the mold, the electromagnetic change-over valves 38, 41, and 45 are made to go into position II. The electromagnetic proportional flow-rate control valve 40 is placed in a fully open state. At this time, the oil inside the oil chamber 35 is supplied to the oil path 42 through the electromagnetic change-over valve 38, the electromagnetic proportional flow-rate control valve 40, and the electromagnetic change-over valve 41, and supplied to the oil chamber 36 again via the electromagnetic change-over valve 38. In this way, a differential circuit operated by pressure areas A.sub.1 and A.sub.2 is formed. The piston 32 moves in the leftward direction in the figure, and a maximum speed for opening the mold can be obtained.
In the hydraulic circuit constructed as described above, when the piston 32 moves, its movement stroke is detected by a displacement sensor 51. A displacement signal from the displacement sensor 51 is sent to a control apparatus 52. The electromagnetic proportional flow-rate control valve 40, and the variable capacity pump 39 are controlled by a control signal from the control apparatus 52.
In the mold opening/closing control apparatus constructed as described above, however, a great shock occurs if a differential circuit is formed while the piston 32 is moving after the mold opening is started. Therefore, the mold opening/closing control apparatus cannot be used for positional control requiring a high degree of precision.